This invention relates to a chain tensioner for keeping constant the tension in a chain such as a camshaft drive chain.
FIG. 9 shows a conventional chain tensioner of this type. It includes a plunger 62 and a tension adjusting spring 63 both mounted in a cylinder chamber 61 formed in a housing 60. The spring 63 biases the plunger 62 outwardly to press it against a chain 64.
Behind the plunger 62 is a pressure chamber 65 to which an oil supply passage 66 opens. A check valve 67 is provided near the oil supply passage 66. When the plunger 62 moves outward so that the pressure in the pressure chamber 65 drops, the check valve 67 opens and simultaneously an oil supply pump is activated to supply hydraulic oil through the oil supply passage 66 into the pressure chamber 65.
When the tension in the chain 64 increases and the plunger 62 is pushed into the housing 60, the hydraulic oil sealed in the pressure chamber 65 serves as a damper, damping the movement of the plunger 62. As the chain 64 slackens, the tension adjusting spring 63 pushes out the plunger 62 quickly to increase the tension in the chain 64. The plunger 62 can smoothly move following fluctuating tension in the chain 64, thus keeping the tension in the chain at a constant level at all times.
In the case of a camshaft drive chain, its tension may increase when the engine is stopped according to the positions of the cams on the camshafts when they are stopped. The chain 64 thus urges the plunger 62 into the housing 60, increasing the oil pressure in the pressure chamber 65. The oil in the chamber 65 thus gradually leaks through a gap present between the sliding surfaces of the plunger 62 and the cylinder chamber 61, thus keeping the plunger 62 retracted.
When the engine is restarted in this state and the chain 64 slackens, the plunger 62 will move a long distance outwardly. Since the engine has just been started, the oil supply pump is not sufficiently warmed up and cannot discharge a sufficient amount of oil into the pressure chamber 65. Thus, air may find its way into the pressure chamber, impairing the damping performance of the chain tensioner.
Also, immediately after the engine has been started, the oil is high in viscosity and low in fluidity. This also is a potential cause of the above problem.
In order to solve this problem, Unexamined Japanese Utility Model Publication 64-25557 proposes to provide the housing with pivotable ratchet pawls to prevent the plunger from being pushed into the housing when the engine is cut, by bringing the ratchet pawls provided on the housing into engagement with a rack provided on the outer periphery of the plunger. But this arrangement has the problem of durability because a load from the chain concentrates on the portion where the rack engages the ratchet pawl. Also, if the teeth on the racks are arranged at a small pitch, the backlash will disappear, causing the chain to be overstretched. If the pitch is large, the backlash will grow large. The chain will thus flap violently when the engine is started until the hydraulic damper is activated.
An object of this invention is to provide a durable, high-quality chain tensioner which has means for preventing air from entering hydraulic oil in the pressure chamber when the fluidity of hydraulic oil is low such as when the engine is started in a cold environment, and which produces less noise.